Rotary machine

ABSTRACT

A rotary machine comprising a stator having magnetic poles each provided with a stator winding, and an armature mounted for rotation within said stator and provided with an armature winding. Each of said magnetic poles is integrally formed at the end thereof with two oppositely directed polepieces which are aligned along the surface of said armature with a gap therebetween, said two polepieces being bisymmetrical so as to provide a larger sectional area of the magnetic path through a trailing one of said two polepieces in the rotating direction of the armature than that through a leading one of said two polepieces, whereby the output capacity of the rotary machine is increased.

i United States Patent [151 3,643,118 'Ichiki et al. 1451 Feb. 15, 1972I 1 ROTARY MACHINE 2,465,824 3/1949 Thomas..... ....310/2s4 72Inventors: Toshinobulchiki; HironoriOkuda, both of 2304334 M955 Bmlsfmd'"310/46 Hm K A lb n f 2,780,741 2/1957 Lynn ....3l0/177 3 3,293,46712/1966 Favre ..310/1s7 .Iapan [73] Assignee: Hitachi, Ltd., Tokyo,Japan Primary Examiner-LT HiX Assistant Examiner-R. Skudy [22] led: 1970AttorneyCraig, Antonelli and Hill 21 A IQN 33289 1 pp 0 57 ABSTRACT Arotary machine comprising a stator having magnetic poles I30] ForelgnApphcauon Monty Dam each provided with a stator winding, and an armatureMay 2, 1969 Japan ..44/33610 mounted for rotation within said stator andprovided with an armature winding. Each of said magnetic poles isintegrally s21 u.s.c1 ..310/4o,310/179,310/258 formed at the end thereofwith two oppositely directed 51 Int. Cl. ..H02k 1/12 Polepieces whichare aligned along the surface of Said [58] Field ofSearch..310/40,46,42,172,173, 174, We Wlth a p therebetween, said twoPolepisces being 310/179, 137, 177, 171 254 256 258 bisymmetrical so asto provide a larger sectional area of the magnetic path through atrailing one of said two polepieces in [56] References Cited therotating direction of the armature than that through a leading one ofsaid two polepieces, whereby the output UNITED STATES PATENTS capacityof the rotary machine is increased. 2,136,301 11/1938 Hoddy ..3l0/40 10Claims, 11 Drawing Figures mas-mover R0721 r/d/v PATENTEDFEB 1 5 I9723,643.1 18' SHEET t 0F 9 INVENTOR TOSHINOEAL IC.HIKI

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ATTORNEYS ROTARY MACHINE This invention relates to rotary machines suchas a commutator motor, commutator generator, DC motor and DC generator,and more particularly to a rotary machine being small in size and havingno commutating pole, which machine is used as a driving source, e.g., inelectrical appliances used for the common home.

The rotary machine of the type has hitherto had its magnetic poles on astator core manufactured bisymmetrically,

and the magnetic paths of the magnetic pole portions tend to have apartiality towards the direction of reversal in a motor and towards thedirection of rotation in a generator under the influence of the armaturereaction which inevitably occurs during operation. Since the armaturereaction causes deterioration in commutation, there has heretofore beenan approa ch in which airgaps near the magnetic pole portions are madelarger or a system in which the cross-sectional area of the magneticpoles near here is made smaller, in order to suppress the armaturereaction to the minimum. With large airgaps. however, the effectivetotal amountof magnetic fluxes will decrease, and in case of machineswith a brush, although improvements in commutation are attained,undesirably the output power will be reduced.

As the magnetic paths in the vicinity of the magnetic poles exhibitpartiality due to the armature reaction, there is a tendency that theiron core is magnetically saturated at some parts of the magnetic poles,while it is not effectively utilized at other parts. With this in mind,the inventors measured magnetic flux at various parts of the stator coreby means of a Hall generator or a search coil, and then calculated themagnetic flux density due to the flux, determined the shapes of thestator cores possessing the most effective utilization factor, and thusattained an increase in the output power without increasing the outsidedimensions. Although any increase in the power will usually become amain cause for deterioration in commutation, the present inventionprovides a rotary machine with a construction free from anydeterioration in commutation.

An object of the invention is to provide a rotary machine with anincreased output power without any increase in external dimensions. 7

Another object is to provide polepieces which are nonbisymrnetric withrespect to the geometrical neutral line of magnetic poles, thereby toincrease the total amount of magnetic flux.

Another object is to modify the configuration of stator yokes, therebyto further increase the total amount of magnetic flux.

Another object is to reduce the number of turns of the stator winding,thus to provide an inexpensive rotary machine.

Another object is to shift the positions of notches in the stator coreto commutate magnetic flux passing from the right and left yokes,whereby the amount of flux is increased and simultaneously theefficiency is improved and whereby the commutation is improved.

The invention is concerned with rotary machines, but the followingdescription will be made with reference to a commutator motor for thesake of convenience. The scope of the invention, however, is notrestricted to the commutator motor, but is applicable to such variousrotary machines as referred to above.

FIG. 1 shows the sectional configuration of a stator core and anarmature core of a prior art commutator motor, the latter core beingrotatably disposed within the former core, with both the cores beingschematically represented while with the outside supporting means, etc.,being omitted from the drawing;

FIG. 2 shows, the operation of the commutator motor constructed as inFIG. I, the state of the distribution of magnetic flux *through thestator core and the armature core, wherein numerals represent the valuesof magnetic flux density at various positions;

FIG. 3 is a graph showing, likewise in the prior art, the magnetic fluxdensity in the gaps between the magnetic poles formed in the stator coreand armature;

FIG. 4 shows in section the configuration of a stator core and anarmature core according to an embodiment of the invention, along withthe path of magnetic flux;

FIG. 5 is a diagram showing the magnetic flux density at variouspositions on the stator core and the armature core;

FIG. 6 is a diagram for comparison of the flux density distributions inthe gaps, between the construction of the inven tion and that of theprior art; and I FIGS. 7 to 11 show several embodiments of theinvention, respectively, in which FIG. 7 is a section of the firstembodiment constituting the essential part of the present invention,

FIG. 8 is a schematic section of the second embodiment,

FIG. 9 is a schematic section of the third embodiment,

FIG. 10 is a schematic section of the fourth embodiment, and

FIG. 11 is a schematic section of the fifth embodiment.

Referring to FIGS. 1 to 3, a prior art construction and the distributionof magnetic flux therein will be described. A represents an armaturecore having [4 teeth, which are respectively designated by numerals l toI4 for convenience sake. S designates a stator core of two poles, whichis integrally provided with magnetic poles P. The extreme ends of themagnetic poles P form pole pieces 15 and 16 bisymmetrical with respectto the geometrical neutral line 00 of the magnetic poles. Y representsyokes of the stator core, and herein the polepieces 15 and 16 aredistinctly termed the polepiece in the direction of rotationand thepolepiece in the direction of reversal, respectively. Numeral 17indicates that position of the yoke Y which is opposed to the polepiece15 in the direction of rotation and which is called ?the opposed core,while numeral 18 designates that position of the yoke Y which is opposedto the polepiece 16 in the direction of reversal and which is alsotermed the opposed core. Numeral 19 represents notches which arerequired in the process of assembly. The yokes Y, the magnetic poles P,the polepieces l5 and 16, the opposed cores I7 and I8, and the notchesl9 referred to hereinabove are usually provided by laminatingsilicon-steel plates which are integrally press worked. The polepiecesl5 and 16, however, may be also magnetically secured to the poles P inthe form of separate pieces. At 20 is designated a stator winding, whichwhen wound on the magnetic poles P finishes the stator. Numeral 21designates an armature winding which is inserted into and disposed inslots 22 formed in the outer periphery of the armature A, and winding ofthe armature winding 21 onto the armature A will complete the armatureA. Numeral 23 refers to slots formed in the stator core S, and in whichthe stator winding 20 is disposed.

As a result of various considerations to attain a furtherminiaturization in the prior art construction thus far described, theinventors decided to measure the utilization factor of the iron cores tostudy whether or not they are effectively utilized as a whole. In themeasurement it is only required to find the flux density at variouspositions, and a Hall generator and a search coil, both being wellknown, were used. The results of the measurements are shown as thenumerical values shown in FIG. 2 indicating flux density at variouspositions, and as per the graph shown in FIG. 3 for the flux density atthe gaps g. It was confirmed as also apparent from FIGS. 2 and 3 thatthe flux density distribution inclines towards the direction of reversaldue to the armature reaction during the running period of the commutatormotor, whereby the stator core is bisymmetric, but nevertheless that thepolepiece portion- 15 the direction of rotation has such low fluxdensities as 0.19, 0.6, and L0 Wb/m. thus not efficiently utilizing theiron core, whereas the polepiece portion 16 in the direction of reversalexhibits such high densities as 1.05, 2.07 and 1.45 Wb/m. thus utilizingthe core under considerably saturated conditions. However, the crosssection of magnetic paths flowing from the side of the pole shoe 15 inthe direction of rotation to the armature A, will extend to-the portionin the direction of reversal and be fairly large on the surface of themagnetic pole P, so that the total amount of magnetic flux increases.Numbers given in FIG. 2 reveal the value of [.76 Wb/m. which is 17percent larger when compared with that of the opposed core 18 in thedirection of reversal of 1.5 Wb/m. It was accordingly made sure that,with the prior art stator core S, apart from the poor utilization factorof the magnetic poles P, problems are also encountered with the designof the yoke Y itself. It was jointly made sure that the pole shoeportion 16 in the direction of reversal is fully saturated rather thanhaving a high flux density, and hence that it is considerably high inmagnetic reluctance. I

According to the resultsof the analysis of these phenomena, it wasexpected that if the magnetic flux density were made uniform, the totalamount of flux would be increased to obtain a commutator motor of goodefficiency and consequently to attain an increase in the output.

In view of the above, it was suggested as an improved construction thatthe gaps on the lower flux density side be made smaller thereby toprovide an easy passage for the flux, whereas the gaps on the higherflux density side were to be formed larger thereby to make the passageof the flux difficult, whereby a uniform distribution of the magneticflux would be attained.

The results obtained, however, were contrary to expectations. And, thecommutator motor became poor in efficiency with the total magnetic fluxreduced and withoutany increase inthe output. This is due to the factthat the side on which the flux density was originally lower is not madeany higher in flux density by narrowing the gaps. More specifically, itis considered that the absolute amount of magnetic flux to pass throughthe gap g is small, as shown in FIG. 2, in that of the tooth portion 1or 2 of the armature A due to an electromotive force in the armaturereaction, and hence the reduction in the width of the gaps g will notincrease the flux density so much,

' whereas the side on which the flux density was higher is subjected toan abrupt decrease in the density and thus the amount of flux' isreduced. In order to increase the total amount of magnetic flux at suchunequal intervals, it is required to make large the cross-sectional areaof all the magnetic paths and to set a small flux density. It was,however, found difficult to increase the amount of flux to a valuehigher than that in the uniform gaps g even when employing suchmeasures, and thus .itwas revealed that an increase in the amountoffluxwith nonuniform gaps g is difficult to attain.

It was confirmed from the above discussions that in order to obtain acommutator motor increased in the total amount of flux for an increasein output power, it is more suitable for accomplishment of the purposeto permit the passage of the most possible flux at portions where theamount offlux is high, rather than to produce a uniform distribution ofthe magnetic fluxes.

Referring again to FIG. 3, the flux distributions at the teeth portions5 and 6 of the armature A are such that the right portions thereof falloff extremely.

This indicates that the polepiece portion 16 in the direction ofreversal in FIG. 1 undergoes magnetic saturation, thereby largelyincreasing the magnetic reluctance. In contrast, the pole shoe portionIS in the direction of rotation has a low flux density, and isdisadvantageously constructed so as to allow an easy passage of leakageflux due to the stator winding 20 and of magnetic fluxes due to thearmature reaction. A construction improved in this point is shown inFIG. 4, wherein the cross-sectional area of the pole shoe portion 16 inthe direction of reversal, which portion is integrally formed in thestator core S, is made large (thick) thereby to provide an easy passagefor the flux, while the cross-sectional area of the pole shoe portion 15in the direction of rotation is rendered small (thin) thereby to providea difficult passage for the flux, whereby the magnetic reluctanceagainst the leakage. flux as well as the flux due to the armaturereaction is increased. The whole construction is as shown by full linesin FIG. 4. FIG. 7 is a drawing for making a comparison between theconstruction of an embodiment as shown in FIG. 4 and the prior artconstruction, the latter being represented by dotted lines in thecomparing figure. With such improved construction, the flux distributionas a whole is as shown by the diagram of magnetic flux in FIG. 4, andthere will be understood the tendency therein that the flux density onthe side of the polepiece 16 in the direction of reversal is madeextremely high. This construction has the flux density measured atvarious positions thereof by the same method as that previouslydescribed and the values are given by numerals (in Wb/mf) in FIG. 5. Asis clear from this figure, the magnetic flux density at the polepiece 15in the direction of rotation and the pole shoe 16 in the direction ofreversal is very high at the latter part 16, but the opposed iron coreportions 17 and 18 exhibit approximately the same degree of magneticflux density, so that the iron cores may be efficiently utilized whenused in a region of appropriate saturation. The distribution of the fluxdensity in the gap portions g with this construction is graphicallyshown by full lines in FIG. 6, in which dotted lines represent thedistribution of the flux density with the prior art machine. As is alsoapparent from this figure, the-flux density which drops to the extremeextent, especially at the part of tooth 5, has been raised as shown bythe full lines, thus increasing the total amount of magnetic flux by I0percent or so. 7

Although an increase in the cross-sectional area of the polepiece 16 inthe direction of reversal will increase the flux due to the armaturereaction, teeth portions 5 and 6 will also approach the magneticsaturation point, so the flux due to the armature reaction are notconsiderably increased and'there was observed a tendency to decrease atthe teeth portions 1 and 7. It is to be understood that when a brush(not shown) is located at the teeth portions 1 and 7 in FIG. 6 (whichportions have a lower flux density than in the symmetric magneticpoles), commutation will be improved over the prior art devices.

Since a single phase small-sized commutator motor generally includes nointerpole, it is common practice to shift the brush location to thedirection of reversal to improve the commutation.

However, when the armature reaction is remarkably strong, the main fluxwill have large variations and hence no flux to produce the requiredelectromotive force for commutation may be obtained. There are two waysto suppress the armature reaction; in one of them, the gaps g arewidened, while in the other, the number of turns of the armature winding2] is reduced. These ways, however, are disadvantageous in thefollowing: With the foregoing construction a commutator motor of thesame capacity and the same number of rotations will be large in size,which is contrary to the commonly desired miniaturization and is notdesirable. v'

According to the construction of an embodiment as shown in FIG. 4,however, with a field magnetomotive force and the amount of flux as inthe prior art, the main flux may be in creased or a furtherminiaturization and reduction in cost may be expected, or by slightlyenlarging the gap width of the gap portions g or decreasing the numberof turns of the armature winding 20, improvements in commutation arestill more advanced.

As a result of studying the flux distribution at the stator core portionS in the prior art construction as shown in FIG. I, it was revealed thatthe opposed core portion 18 has less flux density, than the opposed coreportion 17. Therefore, the cross-sectional area of the opposed core 18may be made smaller than that of the opposed core 17 without anytrouble, and obviously, if the cross-sectional area of the opposed core17 is set slightly larger than in the prior art in order to increase theamount of magnetic flux, a still higher efficiency will be attained.More specifically, the cross-sectional area of the magnetic path flowingfrom the polepiece portion 15 in the direction of rotation into thearmature A is very large, and hence, although the flux density at thepole shoe 15 in the direction of rotation is low, the total amount offlux, which naturally increases the amount of flux passing through theopposed core 17 becomes high. Thus, with a large cross-sectional area ofthe magnetic path of the opposed core 17, still more flux will flow toincrease the output. In contrast, at the opposed core part 18 in thedirection of reversal in spite of the pole shoe portion 16 in thedirection of reversal being magnetically saturated the flux density islow, and hence the iron core is not effectively used. Accordingly, thecross-sectional area of the opposed core portion 18 may be designed tobe somewhat smaller without any detrimental effects. This is a veryadvantageous point, as will be described later, regarding theminiaturization of the machine. More specifically, the space for windingtherein the stator winding 20 or the space of the slot 23 has theminimum possible value. For example, if the crosssectional area of thepolepiece is made large, the space of the slot 23 will become smaller byjust that amount and there will be no other means of settlement thanincreasing the outside dimensions of the yoke Y in order to reasonablywind the stator winding 20. According to the second embodimentof theinvention as shown in FIG. 8, the yoke Y may increase the whole amountof magnetic flux and still may be made nonbisymmetric. Especially, bymaking thick the opposed core 17 on the side of the polepiece in thedirection of rotation whereas by thinly forming the opposed core 18 onthe-side ofthe polepiece 16 which core has a large cross-sectional area,the minimum required space of the slot 23 for therein winding the statorwinding may be insured without any increase in the outside dimensions ofthe stator core S. This will result in an increase in the total amountofflux as previously referred to, and advantageously achieves anincrease in the output. Although a larger cross-sectional area of theopposed core 17 increase leakages flux near the magnetic poles P, thecross-sectional area of the polepiece 15 in the direction of rotationwhich area is set at a small value as shown in FIG. 7, will increase themagnetic reluctance ofthis portion, thus-enabling an increase in theleakage flux to be suppressed. On the contrary, although a largecross-sectional area of the pole shoe 16 in the direction of reversaland as shown in FIG. 3 will similarly increase the leakage flux to theabove description, such increase in the leakage flux may be suppressedby setting a small cross-sectional area for the opposed core 18L It isthus possible to accomplish, as a whole, improvements in the commutationofa commutator motor.

As described above, there are two means to improve the efficiency andthereby to increase the output; one is to render nonbisymmetric thecross-sectional areas of the cores near the magnetic poles p of thestator core S, while the other is to make different the cross-sectionalareas of the polepieces l5 and 16. It is a construction shown in FIG. 9which employs both the means simultaneously, and the construction has afurther increase in the output when compared with any construction inwhich either one means is performed.

In addition, this serves to suppress the magnetic flux due to thearmature reaction, and is much preferred to the desired improvements incommutation. A construction shown by dotted lines in FIG. 9 is one ofthe prior art.

When, as the third embodiment shown in FIG. 10, the notch 19 formed onthe stator core portion S is moved slightly towards the direction ofreversal away from the geometrical neutral line 0-0 of the magneticpoles p, the cross-sectional area of the magnetic path of the opposedcore portion 17 on the side of the direction of rotation will becomelarge. Naturally, the cross-sectional area of the magnetic path of theopposed core portion 18 on the side of the direction of reversal becomessmall. Then, the total amount of flux is increased for the foregoingreasons, and the utilization factor of the stator core S is still moreimproved, whereby an increase in the output may be attained. FlG. 10shows by full lines the construction in which the first and secondconstructional measures are included, and by dotted lines the prior artconstruction.

in some prior art commutator motors, the widths of the gaps g are madeunequal for the purpose of improving commutation. Through combination ofthe unequal gaps and the various constructions embodying the presentinvention, it is possible to obtain a commutator motor which is moreefi'ective in efficiency and in commutation. As is clear in FIG. 2, theflux due to the armature reaction is higher at the teeth portions 1 and2 than at the teeth portions 5 and 6. Accordingly, by making the gaps gnarrow between the polepiece 16 in the direction of reversal and theteeth 5 and 6 and wide between the polepiece 15 in the direction ofrotation and the teeth 1 and 2, the amount offlux on the side of thepolepiece 16 in the direction of reversal is increased to increase thetotal amount of flux thereby to improve the efficiency, andsimultaneously, the magnetic reluctance on the side of the polepiece 15in the direction of rotation is increased to improve the commutatingaction. This construction is given in FIG; 11 as the fourth embodiment.With the prior art construction in which the polepieces are bisynimetricand which employs the unequal gaps 3, it was impossible to expect anincrease in the amount of flux because of the magnetic saturation of thepole shoe portion 16 in the direction of reversal.

It is not preferable from the viewpoint of improving commutation tonarrow the gaps on the side of the polepiece 16 in the direction ofreversal. However, the teeth portions 5 and 6 are magnetically saturatedto prevent the flux due to the armature reaction from flowing, and thegap width on the side of the polepiece 15 in the direction of rotationhas been extended. Improvements in commutation may therefore beattained. This construction is as shown by the full lines in FIG. 11, inwhich dotted lines depict the prior art construction.

With the above-described nonbisymmetric magnetic pole P, their baseportions tend to be widened. For this reason, the stator winding 20wound on the magnetic poles P would prove to be of uneconomic length.Therefore, as shown in FIGS. 9 to 11, the polepiece 15 is formed thinlyas far as possible, and simultaneously, construction is made in such amanner that the bottommost part of the slot portion 23 or point isbrought as near as practicable toward point whereby the stator winding20 is permitted to be short.

Since the copper loss of the stator is in proportion to the length of aconductor for a fixed cross-sectional area, it will be reduced with ashorter length at the same number of turns, thus enabling an increase inthe output power.

Although description has been made of a commutator motor, the inventionmay be applied to a commutator generator, a DC motor, a DC generator,and other rotary machines.

More specifically, since in a generator the magnetic flux is shiftedtowards the direction of rotation under the influence of the armaturereaction, the cross-sectional area of a polepiece on the side of thedirection of rotation is made larger relative to that of a polepiece inthe direction of reversal, while the relation is reversed as to thecross-sectional area of the opposed iron cores. Further constructions ofa generator are reverse in nature to a motor, so that the invention isreadily applicable by reversing the construction of each embodimentthereof.

Various modifications in construction may be made within the scope ofthe appended claims.

What is claimed is:

l. A rotary machine comprising a stator core, a plurality of magneticpoles each having two oppositely extending pole pieces integrally formedat its extreme ends, a stator winding which is disposed in slots formedbetween said magnetic poles and a yoke and which is wound on saidmagnetic poles, an armature rotatably supported in said stator core, andan armature winding disposed in slots of said armature and wound on eachtooth, said polepiece which extends in the direction of reversal of saidarmature having a larger cross-sectional area of the magnetic paththereof relative to that of the other of said polepieces which extendsin the direction of rotation of said armature, the gap between all ofsaid magnetic poles and said armature being substantially uniform.

2. A rotary machine comprising a stator core, magnetic poles each havingone end fixed to said stator core and provided with a stator winding, anarmature rotatably supported in said stator core and having an armaturewinding, wherein each of said magnetic poles is formed at the other endthereof with two oppositely directed polepieces which are alignedInIh'I-I mum along a rotational direction of said armature and faced tosaid armature, the trailing one of said two polepieces in saidrotationaldirection of said armature having a larger cross-sectional area of themagnetic path than the leading one of said polepieces.

3. A rotary machine according to claim 2 wherein a portion of saidstator core opposing to said leading polepiece has a largercross-sectional area of the magnetic path than a portion of the sameopposing to said trailing polepiece.

4. A rotary machine according to claim 2 wherein said stator core isformed with notches at portions on its outer periphery corresponding tosaid magnetic poles, each of said notches being slightly deviated fromthe geometrical centerline of a corresponding one of said magneticpoles.

S! A rotary machine according to claim 2 wherein said two polepieces areopposing to said armature'with a gap, said gap being gradually widenedalong the rotational direction.

6. A rotary machine according to claim 3 wherein said stator core isformed with notches at portions on its outer periphery corresponding tosaid magnetic poles, each of said notches being slightly deviated in therotational direction from the geometrical center line of a correspondingone of said magnetic poles.

7. A rotary machine according to claim 2 further comprising brushespositioned on portions of said armature where the flux density of themagnetic field therethrough are substantially a minimum.

8. A rotary machine as defined in claim 1 wherein the opposed coreadjacent the polepiece which extends in the direction of rotation of thearmature has a cross-sectional area which is larger relative to thecross-sectional area of the opposed core adjacent the pole piece whichextends in the direction of reversal of the armature.

9. A rotary machine as defined in claim 1 wherein notches are formed onthe outer peripheral portions of said magnetic poles at positions spacedfrom the line of symmetry of said poles in the direction of reversal ofthe armature.

10. A rotary machine as defined in claim 1 wherein a gap is formedbetween each pole piece and the adjacent opposed core, the gaps formedby the polepieces extending in the direction of rotation of the armaturehaving a larger width than the gaps formed by the polepieces extendingin the direction of reversal of the armature.

1. A rotary machine comprising a stator core, a plurality of magneticpoles each having two oppositely extending pole pieces integrally formedat its extreme ends, a stator winding which is disposed in slots formedbetween said magnetic poles and a yoke and which is wound on saidmagnetic poles, an armature rotatably supported in said stator core, andan armature winding disposed in slots of said armature and wound on eachtooth, said polepiece which extends in the direction of reversal of saidarmature having a larger cross-sectional area of the magnetic paththereof relative to that of the other of said polepieces which extendsin the direction of rotation of said armature, the gap between all ofsaid magnetic poles and said armature being substantially uniform.
 2. Arotary machine comprising a stator core, magnetic poles each having oneend fixed to said stator core and provided with a stator winding, anarmature rotatably supported in said stator core and having an armaturewinding, wherein each of said magnetic poles is formed at the other endthereof with two oppositely directed polepieces which are aligned alonga rotational direction of said armature and faced to said armature, thetrailing one of said two polepieces in said rotational direction of saidarmature having a larger cross-sectional area of the magnetic path thanthe leading one of said polepieces.
 3. A rotary machine according toclaim 2 wherein a portion of said stator core opposing to said leadingpolepiece has a larger cross-sectional area of the magnetic path than aportion of the same opposing to said trailing polepiece.
 4. A rotarymachine according to claim 2 wherein said stator core is formed withnotches at portions on its outer periphery corresponding to saidmagnetic poles, each of said notches being slightly deviated from thegeometrical centerline of a corresponding one of said magnetic poles. 5.A rotary machine according to claim 2 wherein said two polepiecEs areopposing to said armature with a gap, said gap being gradually widenedalong the rotational direction.
 6. A rotary machine according to claim 3wherein said stator core is formed with notches at portions on its outerperiphery corresponding to said magnetic poles, each of said notchesbeing slightly deviated in the rotational direction from the geometricalcenter line of a corresponding one of said magnetic poles.
 7. A rotarymachine according to claim 2 further comprising brushes positioned onportions of said armature where the flux density of the magnetic fieldtherethrough are substantially a minimum.
 8. A rotary machine as definedin claim 1 wherein the opposed core adjacent the polepiece which extendsin the direction of rotation of the armature has a cross-sectional areawhich is larger relative to the cross-sectional area of the opposed coreadjacent the pole piece which extends in the direction of reversal ofthe armature.
 9. A rotary machine as defined in claim 1 wherein notchesare formed on the outer peripheral portions of said magnetic poles atpositions spaced from the line of symmetry of said poles in thedirection of reversal of the armature.
 10. A rotary machine as definedin claim 1 wherein a gap is formed between each pole piece and theadjacent opposed core, the gaps formed by the polepieces extending inthe direction of rotation of the armature having a larger width than thegaps formed by the polepieces extending in the direction of reversal ofthe armature.